Also called the Fenian Cycle--for Fionn's warrior band, the Fianna--the "Fionn Cycle" traces the exploits of Fionn mac Cumhaill, the wandering, woodland warrior of Irish myth. They were popular in both Ireland and Scotland, each country producing numerous narratives and poems on the subject.

It is felt that the Fionn Cycle portrays remnants of Neolithic and Bronze Age life--they spend the summer half of the year hunting, and the winter half in civilization. Here, civilization means the court of King Cormac Mac Airt--for the Fiana are Cormac's mercenaries, often doing battle for him, or for personal vendettas. Fionn even marries Cormac's daughter Grainne, but loses her to his nephew Diarmud, all of which is reminiscent of the story of Tristan and Iseult, with Fionn in the role of King Mark.

The second main story is that of Fionn's son Oisin, who is taken off to Tír na nÓg by a fairy woman, only to return after hundreds of years and find the Fiana and all the heroes dead and replaced by Saint Patrick and his missionaries. There is a long narrative poem--The Colloquy of the Old Men--based on this, in which Saint Patrick asks Oisín about the heroic, pagan era of Ireland, which has now given way to the Middle Ages and Christianity.

One final thing to keep in mind. The names of Fionn and his family: Fionn's original name Demne, his first wife Sabd, his son Oisin, and grandson Oscar, all have names which have elements of the deer in them:

Fionn may have originally been a sort of zoomorphic god, a type of Cernunnos, the deer-horned god in Gaul. Moreover, it is Fionn who contributes important elements of Celtic belief in wisdom through his eating of the salmon of wisdom, and it's related theme of imbas forosnai.

Stories:

The Cause of the Battle of Cnucha

The Boyhood Deeds of Fionn

How Fionn Found Knowledge

Fionn and the Man in the Tree

The Pursuit of Diarmuid and Grainne

The Hiding of the Hill of Howth

The Little Brawl at the Hill of Allen

The Fairy-Palace of the Rowan Trees

The Battle of Fionntragha (Ventry)

The Battle of Gabhra

The Feast at Conan's House

The Adventure of the Churlish Clown in the Grey-Drab Coat

The Pursuit of the Hard Gilly

The Death of Fionn

Oisin in the Land of Youth

The Colloquy of the Old Men

The narratives can be found in both Ancient Irish Tales, translated by Cross & Slover, and Old Celtic Romances translated by P.W. Joyce.

Aside from these narrative tales, there were also poems annonymously composed and put in the names of several characters; it was translated as Duanaire Finn--The Lays of Finn in two volumes: the first in 1908 by Eion Mac Neill, and the second in 1933 by Gerard Murphy. These poems are taken from a single manuscript written in about 1626, but drawing on older material.
The poems are as follows:

Vol. I:

"The Abduction of Eargna"

"Fionn's Foray to Tara"

"The Rowan-Tree of Clonfert"

"The Battle of Cronnmhoin"

"The Bathing of Osin's Head"

"The Fray at Loch Luig"

"Caoilte's Mischief-Making"

"The Crane-Bag"

"Goll's Malediction"

"Goll's Parting With His Wife"

"The Kindred of Fionn"

"The Household of Almha"

"The Headless Phantoms"

"The Enchanted Stag"

"The Boyhood of Fionn" (an abridgement of "The Boyhood Deeds")

"The Shield of Fionn"

"Caoilte's Urn"

"The Daughter of Diarmaid"

"Lament for the Fiana"

"The Sword of Oscar"

"The Battle of the Sheaves"

"The Death of Goll"

"The Adventure of the Men from Sorcha"

"The Chair of Sliabh Truim"

"Once I Was Yellow-haired"

"Woe for Them that Wait on Churchmen"

"Oisin's Sorrow"

"Three Heroes Went We to the Chase"

"Erect Your Hunting Spear"

"The Hunger of Crionloch's Church"

"The Wry Rowan"

"The Beagle's Cry"

"The Sleep-song For Diarmaid"

"Fionn's Prophecy"

"The War-vaunt of Goll"

Vol. II:

"The Lay of the Smithy"

"Fionn's Ancestry"

"The Naming of Dun Gaire"

"The Battle of Gabhair"

"These Six"

"The Bird-Crib"

"The Standing Stones of Ireland"

"The Womenfolk of the Fian"

"Lugh's Kinship with Certain Members of the Fian"

"The Kinship of Cnu Dhereoil with Fionn"

"The Kinship of Fiamhoin son f Forach with Oisin"

"Caoilte's Sword"

"The Wild Rush of the House of Morna"

"Fionn's Prophecy"

"The House of Morna Defend Fionn in Hell"

"Rise Up, Oisin"

"Rise Up, Oscar"

"The Bell on Druim Deirg"

"The Magic Pig"

"Oisin in Elphin"

"Bran's Departure from the Fian"

"The Dialogue of Oisin and Patrick"

"The Chase of Slievenamon"

"The Coming of Laighne Mor"

"The Chase Above Lough Derg"

"Manannan and the Fian"

"The Adventure on Slieve Gullen with Dugh son of Diorfadh and the Prowess of Oscar"

"The Coming of the Dearg, Son of Droicheal"

"Goll's Tomb and the Coming of Magnus the Great"

"The Magic Cloak"

"Fionn's Foray to Tara" (same as Vol. I No. 2, but with some added material)

"The Lay of Airreghean the Great, Son of Ancar"

"The Lay of Beann Ghualann"

"The Chess-Gaem Beneath the Yew-tree"

There is also the folktale of "The Legend of Knockmany" which uses the names of Fionn and Cuchulainn as two giants who do battle. Finn is able to trick Cucullin into losing his golden finger. This is also the origin of the story that Finn built the Giants' Causway so as to intimidate Cucullin. Of course, the names of these two giants have very little to do with two Irish heroes who are supposed to have lived some two hundred years apart.